Self-steering arrays are promising substitutes for expensive smart antenna systems in simple line-of-sight communication environments. One of the most popular self-steering array technologies is the retrodirective array, which automatically sends a signal back to an interrogating source, without any phase shifters or digital signal processing, as referenced in the overview by R. Y. Miyamoto and T. Itoh, “Retrodirective arrays for wireless communications,” IEEE Microwave Magazine, pp. 71-79, March 2002. The enhanced communication link between the retrodirective array and interrogator arising from this self-steering feature makes it useful for multi-user communications schemes such as space division multiple access (SDMA) or multi-transponder (satellite) networks. To increase user capacity in SDMA, interception and interference is minimized by increasing the array directivity, implying that a large number of array elements are needed. However, this increases the size and cost of the system.
There have been many studies to effectively increase the directivity of an array without increasing the number of elements. One example is null steering, which is often used in monopulse radar systems and direction-of-arrival (DOA) estimation algorithms, as a null can provide much higher resolution than a beam. While these techniques have typically been used in receivers, secure data exchange systems require high directivity for transmission as well.
Other techniques to obtain high directivity for transmission have been developed in different applications. U.S. Patent Application 2003/0231700 to Alamouti et al. discloses a phased steering array of vertically spaced antennas to give vertical spatial adaptivity to a wireless discrete multitone spread spectrum (DMT-SS) communications system. This enables the automatic positioning of a beam in the vertical direction to position nulls where interferers are located on the same azimuth, but separated in elevation. U.S. Patent Application 2003/0123384 to Agee discloses a “stacked-carrier” spread spectrum (SCSS) communication system which is combined with retrodirective array, code-nulling, and interference canceling techniques. For example, soft nulls of the transmitted signal can be directed toward interference sources, and code-nulling and retrodirective beam steering techniques can be combined in adaptive antenna arrays to improve the range of a conventional transceiver, and also to increase the capacity of a communications network by allowing tighter spatial packing. U.S. Patent Application 2003/0086366 to Branlund et al. discloses an adaptive communications method for multi-user packet radio wireless networks using code preambles to identify signals from/to remote units.
U.S. Pat. No. 6,480,522 to Hoole et al. (AT&T) discloses a stacked carrier DMT-SS communication method based on frequency domain spreading, and including code-nulling techniques for interference cancellation and enhanced signal separation. U.S. Pat. No. 5,781,845 to Dybdal et al. (Aerospace Corp.) discloses an adaptive transmitting antenna array which can adjust phased weighting coefficients to produce soft nulls in the direction of reflective objects which would otherwise cause multipath distortions. U.S. Pat. No. 4,849,764 to Heyningen (Raytheon) discloses an adaptive antenna array which uses an inverse beamformer to subtract out signal components from the direction of an interfering signal to enhance reception from a target signal source. U.S. Pat. No. 4,641,259 to Shan et al. (Stanford U.) discloses an adaptive antenna array which uses a smoothing operation to obtain a feedback signal to suppress interfering signals. U.S. Pat. No. 4,246,585 to Mailloux (USAF) discloses an adaptive antenna array which uses deterministic and adaptive null steering for improved null control of the signal beam. U.S. Pat. No. 4,107,609 to Gruenberg discloses a communication transponder system which uses two antenna arrays, a first array of which receives a signal from a first station direction and transmits a second signal with a null in the first station direction, and a second array coupled to the first array which transmits a beam peak of the second signal in a second station direction.
However, despite these developments in the prior art, it would be desirable to provide a secure data transmission method for microwave self-phasing antenna arrays wherein the signal-to-noise ratio (SNR) for data transmission toward a target can be maximized while minimizing interception in other directions. It would also be desirable to achieve this objective without the complexities associated with the prior art.
In a related area, there has been considerable interest in small satellites for various applications as described, for example, by H. Heidt, et al., “CubeSat: a new generation of picosatellite,” in Proc. of the 14th Annual AIAA/USU Conference on Small Satellites, Logan, Utah, August 2001. The smaller mass of nanosatellites (10 kg) and picosatellites (1 kg) make them more economical to develop and launch into orbit. Networks of small satellites promise increased mission flexibility and success by distributing the tasks and subsystems typical of a single large satellite. An autonomous small-satellite network also reduces the possibility of catastrophic single-point failure; if one small-satellite fails, others can take up the slack until a replacement is launched. However, the challenge in designing a distributed small-satellite network—especially a dynamically reconfigurable one—is in establishing and maintaining a reliable crosslink with other satellites in the network without a priori knowledge of their positions.
Omnidirectional antennas are the obvious choice for crosslinking satellites that are subject to constant repositioning, but this leaves the network susceptible to eavesdropping by unauthorized ground stations as well as by satellites outside the network. Omnidirectional antennas are also inefficient, as power is radiated in all directions, not just in the direction of the receiver. In covert or security-sensitive networks, signal interception can be prevented by employing direct crosslinks with conventional phased-array antennas. However, for picosatellites in the 1000- to 1500-cubic-cm range, processing power is a valuable resource and dynamic beam steering would add another layer of complexity to the system, negating the advantages of the simple, low-cost features of these small satellites. It may also be desirable to provide for secure data transmission where the signal-to-noise ratio toward a target is maximized while minimizing interception in all other directions.